Conventional golf balls can be divided into two general classes: solid and wound. Solid golf balls include one-piece, two-piece (i.e., solid core and a cover), and multi-layer (i.e., solid core of one or more layers and/or a cover of one or more layers) golf balls. Wound golf balls typically include a solid, hollow, or fluid-filled center, surrounded by a tensioned elastomeric material, and a cover. It is also possible to surround a hollow or fluid-filled center with a plurality of solid layers. Solid balls have traditionally been considered longer and more durable than wound balls, but many solid constructions lack the “feel” provided by the wound construction.
More recently, by altering ball construction and composition, manufacturers have been able to vary a wide range of playing characteristics, such as compression, velocity, “feel,” and spin, optimizing each or all be optimized for various playing abilities. In particular, a variety of core and cover layer(s) constructions, such as multi-layer balls having dual cover layers and/or dual core layers, have been investigated and now allow many non-wound balls to exhibit characteristics previously maintainable in a solid-construction golf ball. These golf ball layers are typically constructed with a number of polymeric compositions and blends, including polybutadiene rubber, polyurethanes, polyamides, and ethylene-based ionomers.
Ionomers, and in particular ethylene α,β-ethylenically unsaturated carboxylic acid copolymers or a melt processable ionomer thereof, are a preferred polymer for many golf ball layers. One problem encountered with the use of ionomers as stiff layers, however, is the unprocessability of the material as the percent of neutralization of the acid group increases. Ionomers are stiffened by increasing the amount of neutralization by a metal cation or a salt thereof. Once the percent of neutralization is greater than about 60% (depending on metal cation selected), the melt flow of the ionomer becomes too low and the ease of processability decreases or disappears altogether. For tri-valent cations, the percent neutralization at which the polymer becomes unprocessable can be significantly lower.
Conventional maleic anhydride polymers, such as the FUSABOND® series, to date have a very low level of maleic anhydride (i.e., less than 1 wt % for the ethylene based polymers and less than 2 to 3 wt % for propylene based polymers). Higher levels of maleic anhydride are difficult or near impossible to achieve in the post polymerization reactive extrusion process due to excessive degradation of the base polymer and/or availability of sufficient active grafting sites. Hence, only a small amount of ionic cross-linking can be accomplished using the low levels of maleic anhydride grafted polymers in comparison to commercially-available ionomers, such as ethylene-methacrylic acid or acrylic acid copolymers, which typically have an acid content in the range of 5 to 20 wt %. Also, achieving levels higher than 20 wt % of the acid monomer in the acid copolymer is difficult due to problems encountered in the copolymerization process fouling of materials, etc.). These limitations affect the ionic cross-linking level and, therefore, the performance of the golf ball components (i.e., COR, abrasion resistance, etc.).
The present invention addresses these deficiencies by neutralizing an alternate copolymer of ethylene-maleic anhydride or their maleic acid moieties having a very high level of maleic anhydride, typically in the range of up to 78 wt %, preferably about 78 wt %. A suitable flow modifier based on an organic acid or their salts, such as fatty acids/salts, or even polymeric flow modifiers, such as thermoplastic elastomers, can be used during the neutralization process in order to accomplish sufficient melt flow characteristics suitable for golf ball manufacturing processes.